Auto Questions: How Power Brakes Work

About the braking system and safety features in modern automobiles with a description of simple mechanism and the more advanced power braking system.

The braking system on an automobile is one of the most important safety features available to drivers. As a result, car manufacturers have devoted extensive years of research to constantly develop and refine these systems to make cars safer. Power assisted brakes were one of these developments and are now found on every modern passenger vehicle made. Let's take a look at how they work.

The braking system on a car is activated when the driver depresses the brake pedal located in the passenger cabin. The pedal is directly linked to a piston in the master cylinder located in the engine bay. The master cylinder is connected to the brakes at each wheel through a series of lines and hoses filled with brake fluid. As fluids are not easily compressed, when pressure is applied to the piston in the master cylinder, the resulting hydraulic pressure is conveyed throughout the entire system. This pressure is ultimately used to squeeze the brake pads/shoes against the discs/drums and slow the vehicle down.

This in essence, is how older braking systems operated. However, with automotive advancements throughout the decades, the weight and speeds of passenger vehicles have increased tremendously. As a result, the pressure a driver could apply using their foot became extremely inadequate to retard a vehicle's speed.



Power brakes are made possible through the use of a vacuum booster mounted on the firewall in the engine bay. The booster looks like a large canister and is located directly behind the master cylinder. The booster is able to magnify the force exerted on the brake pedal through the use of vacuum assistance created during the engine's operation. As air is drawn into an engine, a vacuum is created in different regions, and manufactures have cleverly utilized this property in developing vacuum assisted power brakes.

A vacuum line runs between the engine and brake booster and a check valve keeps vacuum pressure from escaping out of the booster housing. The booster is an empty canister that has two internal chambers separated by a central rubber diaphragm. When the brake pedal is NOT being applied, a valve in the center of the diaphragm remains open to allow a vacuum to be created in both chambers.

When the driver pushes the brake pedal, the valve in the diaphragm closes, and another one opens to allow air into the vacuum chamber closest to driver, called the control chamber. The great pressure created as air rushes into the control chamber is used to push against the center diaphragm, which in turn applies force to the piston in the master cylinder. This additional force magnifies the pressure the driver applies to the brake pedal, which in turn magnifies the hydraulic pressure exerted throughout the braking system.

This is the way in which vacuum assisted power brakes operate to retard a vehicle's speed, and as a result of their introduction, braking distances on modern automobiles have reduced significantly, while simultaneously reducing the effort needed from the driver to stop a vehicle.

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